1. Field of the Invention
The present invention relates to a process for the preparation of a sintered silicon nitride body having a surface compression strength and hence, being excellent in the strength at high temperatures. More particularly, the present invention relates to a process for preparing a sintered silicon nitride body having an improved surface texture by hot isostatic pressing. Furthermore, the present invention relates to a novel sintered silicon nitride body prepared according to this process.
2. Description of the Prior Art
Recently, the HIP process (hot isostatic pressing sintering process) has attracted attention as means for sintering a ceramic material to densify the ceramic material with use of a small amount of a sintering aid, and various treating techniques utilizing this HIP process have been developed. Especially for obtaining a sintereed boby having a complicate shape from a molded powder, there is mainly adopted a so-called glass capsule process in which a powder, a green molded body or a pre-sintered body is filled in a glass (Vycor or Pyrex) vessel and is preliminarily sintered under compression by an inert gas at a temperature close to the softening point of the glass vessel to cover the periphery of the compressed body with the glass, the interior powder, green molded body or pre-sintered body is densified in a high-temperature high-pressure gas by using the covering glass layer as a pressure-transmitting medium, and the intended sintered body is obtained by removing the glass from the periphery of the formed sintered body by using a dissolving solution or according to the sand blasting method.
This process, however, is defective in that since a large quantity of the softened glass component intrudes in the grain boundary phase in the sintered body, the strength at high temperatures is drastically reduced.